Liquid crystal apparatus

ABSTRACT

A liquid crystal apparatus includes: a liquid crystal device comprising a group of first electrodes, a group of second electrodes intersecting the first electrodes, and a ferroelectric liquid crystal disposed between the group of first electrodes and the group of second electrodes so as to form a picture area comprising a pixel at each intersection of the first and second electrodes; and drive means for applying a scanning selection signal to the first electrodes N electrodes apart (N: a positive integer), and applying data signals through the second electrodes to all or a prescribed part of the pixels on a particular first electrode under application of the scanning selection signal so as to first form a dark state at said all or a prescribed part of the pixels on the particular first electrode and then form a bright state at a selected pixel among said all or a prescribed part of the pixels on the particular first electrode.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to a liquid crystal apparatus,particularly one using a ferroelectric liquid crystal.

Clark and Lagerwall have disclosed a surface-stabilized bistableferroelectric liquid crystal in Applied Physics Letters, Vol. 36, No. 11(Jun. 1, 1980), p.p. 899-901, and U.S. Pat. Nos. 4,367,924 and4,563,059. The bistable ferroelectric liquid crystal has been realizedby disposing a chiral smectic liquid crystal between a pair ofsubstrates which are set to provide a spacing small enough to suppressthe formation of a helical arrangement of liquid crystal moleculesinherent to the bulk chiral smectic phase of the liquid crystal andaligning vertical molecular layers each composed of a plurality ofliquid crystal molecules in one direction.

A display panel comprising such a ferroelectric liquid crystal may bedriven by a multiplexing drive scheme as disclosed by, e.g., U.S. Pat.No. 4,655,561 to Kanbe, et al., to provide a display with a large numberof pixels.

A ferroelectric liquid crystal as described above shows a responsivetime which depends on the surrounding temperature, so that a drivingpulse duration at a lower temperature is required to be longer than at ahigher temperature. As a result, a drive frequency for forming onepicture (frame frequency) is lowered at a lower temperature andgenerally lowered to a frame frequency as low as 1-30 Hz. For thisreason, a display at a lower temperature is liable to cause "flickering"to provide a display image of a poor display quality.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a liquid crystalapparatus having solved the above-mentioned problems, particularly theoccurrence of flickering.

According to the present invention, there is provided a liquid crystalapparatus, comprising:

a liquid crystal device comprising a group of first electrodes, a groupof second electrodes intersecting the first electrodes, and aferroelectric liquid crystal disposed between the group of firstelectrodes and the group of second electrodes so as to form a picturearea comprising a pixel at each intersection of the first and secondelectrodes; and

drive means for applying a scanning selection signal to the firstelectrodes N electrodes apart (N: a positive integer), and applying datasignals through the second electrodes to all or a prescribed part of thepixels on a particular first electrode under application of the scanningselection signal so as to first form a dark state at said all or aprescribed part of the pixels on the particular first electrode and thenform a bright state at a selected pixel among said all or a prescribedpart of the pixels on the particular first electrode.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an apparatus according to the presentinvention.

FIG. 2 is a schematic plan view of a matrix electrode structure used inthe present invention.

FIG. 3 shows a set of drive signal waveforms for multiplexing drive usedin the present invention, and FIG. 4 shows a drive signal waveform of acomparative scanning selection signal.

FIGS. 5 and 7 respectively show another set of drive signal waveformsfor multiplexing drive used in the present invention.

FIG. 6 is a schematic plan view of another matrix electrode structureused in the present invention.

FIGS. 8 and 9 are schematic perspective views for illustratingferroelectric liquid crystal cells used in the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of a liquid crystal apparatus according to thepresent invention. The apparatus includes a liquid crystal display panel11 for providing a picture area or screen which comprises an imagedisplay area 11A for forming an image depending on data signals and amarginal region 11B which is a non-display region for not displaying animage. The liquid crystal display panel 11 is constituted by aferroelectric liquid crystal and is provided with a drive unit thereforcomprising a scanning drive circuit 12 and a data/margin drive circuit13 which may in turn comprise a data drive circuit 13A and a margindrive circuit 13B. The image display region 11A may be driven by thescanning drive circuit 12 and the data drive circuit 13A and themarginal region(s) 11B may be driven by the scanning drive circuit 12and the margin drive circuit 13B. Referring also to FIGS. 2 and 3, thescanning drive circuit 12 supplies scanning signals S₁, S₂, S₃, . . . ,and the data/margin drive circuit 13 supplies data signals I₁, I₂, I₃, .. . and data signals for marginal display W₁, W₂, W₃ . . . The scanningdrive circuit 12 and the data/margin drive circuit 13 are respectivelyaddressed by an address decoder 14, and the data electrodes for applyingdata signals for marginal display 23 are also designated by the addressdecoder 14. Further, column data 16 are controlled by a CPU 15 andsupplied to the data/margin drive circuit 13 so as to effect an imagedisplay in the image display region 11 and provide a uniformly bright ordark optical state at the marginal region 11B.

FIG. 2 illustrates a matrix electrode structure disposed on the liquidcrystal display panel 11. In the image display region 11A in the liquidcrystal display panel or picture area 11, pixels formed at theintersections of the scanning electrodes 21 and the data electrodes 22are arranged in X rows and Y columns (X: number of scanning electrodesand Y: number of data electrodes), and in the marginal region(s) 11B,pixels formed at the intersections of the scanning electrodes 21 and theelectrodes for marginal display 23 are arranged. The number of theelectrodes for marginal display 23 should be determined so as to providethe marginal region with an appropriate width which may be severalmilli-meters to several centimeters.

Between the scanning electrodes 21 (first group) and the data electrodes22 and electrodes for marginal display 23 (second group), aferroelectric liquid crystal is disposed so as to provide a bright state(L) and a dark state (D) through application of driving signal waveformsas shown in FIG. 3.

According to a driving embodiment shown in FIG. 3, in a scanningselection period (in which a scanning selection signal is to be appliedfor selection of a scanning electrode) including a sub-period T₁ and asub-period T₂, the pixels on a selected scanning electrode aresimultaneously cleared into a dark optical state ("D" or black "B") inthe period T₁ and a pixel selected therefrom is selectively switchedinto a bright optical state ("L" or white "W"). While the othernon-selected pixels retain the dark optical state to effect writing on ascanning electrode. The above operation is repeated N electrodes apart(two lines apart, i.e., every third line, in this embodiment) in oneseries of scanning (one field scanning), and N+1 series of scanning(three times of field scanning in this embodiment) are performed tocomplete one cycle of scanning (one frame scanning) thereby forming onepicture corresponding to given data signals. In the above-mentioneddrive mode for display, cross nicol polarizers may be adjusted to setthe optical state in the period T to be a dark state. In this instance,the frequency of the field scanning may be set to 20 Hz or higher,preferably 30 Hz or higher.

In the image display region 11A, an image is displayed depending ongiven data signals applied to the data electrodes 22. Further, theelectrodes for marginal display are controlled so as to provide a bright(white) optical state uniformly at the pixels in the marginal region 11Bwhile not shown in the figure.

Then, a liquid crystal panel having the following dimensions wassubjected to image display according to the following Modes 1 and 2.

Liquid crystal panel

Ferroeletric liquid crystal: "CS-1017" (trade name, available fromChisso K. K.)

Cell gap: 1.5 micron

Number of scanning electrodes: 400

Number of data electrodes: 640

Mode 1

One scanning period: 180 μsec

Drive voltages:

±V_(S) =±18 V

±V_(I) =±6 V

Temperature: 25° C.

Mode 2

One scanning period: 400 μsec

Drive voltages:

±V_(S) =±15 V

±V_(I) =±5 V

Temperature: 15° C.

The image forming operations according to the above mentioned Modes 1and 2 were performed with skipping of different numbers of scanningelectrodes and respectively subjected to evaluation by a panel composedof arbitrarily selected panelists. The results are summarized in thefollowing Table 1 wherein ⊚ denotes a case where all 20 panelistsrecognized no flickering: o, 15-19 panelists recognized no flickering:Δ, 15-19 panelists recognized flickering; and x, 20 panelists recognizedflickering.

                                      TABLE 1                                     __________________________________________________________________________    Mode                                                                          __________________________________________________________________________         N (scanning N                                                                         0  1  2   3  4   5  6   7                                             lines apart)                                                                  Spatial 6.3                                                                              12.6                                                                             18.9                                                                              25.2                                                                             31.5                                                                              37.8                                                                             44.1                                                                              50.4                                          frequency (Hz)                                                           1    Evlauation                                                                            Δ                                                                          ⊚                                                                 ⊚                                                                  ⊚                                                                 ⊚                                                                  ⊚                                                                 ⊚                                                                  ⊚                              of flickering                                                            2    Evaluation                                                                            x  Δ                                                                          ∘                                                                     ⊚                                                                 ⊚                                                                  ⊚                                                                 ⊚                                                                  ⊚                              of flickering                                                            __________________________________________________________________________

From the above results, it has been found that an image display freefrom flickering could be realized even at a low temperature, if thenumber N of skipped scanning electrodes was two or more, preferablythree or more. No flickering was observed either in the marginal regions11B.

Next, as a comparative test, the above-mentioned image formationaccording to Mode 2 was repeated except that a scanning selection signalshown in FIG. 4 was used instead of the scanning selection signal shownin FIG. 3 (as a result, simultaneous erasure into a bright state wasperformed in a period t₁ corresponding to T₁ in FIG. 3 and selectivewriting into a dark state was performed in a period t₂ corresponding toT₂ in FIG. 3). The results of evaluation are summarized in the followingTable 2 according to the same standards as in Table 1.

                  TABLE 2                                                         ______________________________________                                        Mode 2                                                                        ______________________________________                                        N (scanning N                                                                           0     1      2    3    4    5    6    7                             lines apart)                                                                  Spatial   6.3   12.6   18.9 25.2 31.5 37.8 44.1 50.4                          frequency (Hz)                                                                Evaluation                                                                              x     x      x    Δ                                                                            ∘                                                                      ⊚                                                                   ⊚                                                                   ⊚              of flickering                                                                 ______________________________________                                    

As shown in Table 2, flickering was much more noticeable than in thedriving according to the driving waveforms shown in FIG. 3. In thiscomparative experiment, in addition to flickering, a fringe patternformed by portions with different luminances occurred in parallel withthe scanning lines in the cases of scanning selection four or more linesapart. This provided a poor display quality in a different sense fromflickering.

FIG. 5 is a waveform diagram showing another set of driving signalwaveforms used in another driving embodiment which is the same as theone explained with reference to FIG. 3 except that different waveformsof scanning selection signal and data signals are used (and also theorder of data signals is arbitrary). In FIG. 5, data signals applied tothe electrodes for marginal display are also shown.

FIG. 6 shows another embodiment of a matrix electrode structure for usein the present invention. In the embodiment shown in FIG. 6, anelectrode for marginal display 23 having a larger width (preferably,several multi-meters to several centi-meters) than the width (generally100-500 microns) of a data electrode 22, is used as electrodes W₁ and W₂in the marginal regions 11B. As a result, the number of terminals can beremarkably decreased as compared with the embodiment shown in FIG. 2,whereby the IC designing for the data/margin drive circuit can besimplified.

Further, as a wider electrode for marginal display 23 is used, thecapacitance for one electrode 23 is increased and a sufficiently largevoltage may be required so as to exceed the threshold voltage of theliquid crystal layer. Accordingly, in a preferred driving embodimentusing an electrode embodiment as shown in FIG. 6, a voltage signalhaving a duration T_(x) which is longer than a maximum pulse duration T₀of a data signal, may be used in synchronism with a scanning selectionsignal. A representative driving waveform example for this embodiment isshown in FIG. 7.

In a driving embodiment shown in FIG. 7, the scanning electrodes 21 anddata electrodes 22 are driven similarly as in the embodiment shown inFIG. 5, but a voltage signal applied to an electrode for marginaldisplay 23 has a pulse duration T_(x) which is 3/2 times a maximum pulseduration T₀ of a data signal I₁, I₂ . . . By applying such a broad pulsevoltage signal to the electrode for marginal display 23, the marginalregion 11B can be securely controlled to a uniform bright state.

Referring to FIG. 8, there is schematically shown an example of aferroelectric liquid crystal cell. Reference numerals 81a and 81b denotesubstrates (glass plates) on which a transparent electrode of, e.g., In₂O₃, SnO₂, ITO (indium-tin-oxide), etc., is disposed, respectively. Aliquid crystal of an SmC*-phase in which liquid crystal molecular layers82 are oriented perpendicular to surfaces of the glass plates ishermetically disposed therebetween. A full line 83 shows liquid crystalmolecules. Each liquid crystal molecule 83 has a dipole moment (P.sub.⊥)84 in a direction perpendicular to the axis thereof. When a voltagehigher than a certain threshold level is applied between electrodesformed on the base plates 81a and 81b, a helical or spiral structure ofthe liquid crystal molecule 83 is unwound or released to change thealignment direction of respective liquid crystal molecules 83 so thatthe dipole moment (P.sub.⊥) 84 are all directed in the direction of theelectric field. The liquid crystal molecules 83 have an elongated shapeand show refractive anisotropy between the long axis and the short axisthereof. Accordingly, it is easily understood that when, for instance,polarizers arranged in a cross nicol relationship, i.e., with theirpolarizing directions crossing each other, are disposed on the upper andthe lower surfaces of the glass plates, the liquid crystal cell thusarranged functions as a liquid crystal optical modulation device ofwhich optical characteristics vary depending upon the polarity of anapplied voltage. Further, when the thickness of the liquid crystal cellis sufficiently thin (e.g., 1 micron), the helical structure of theliquid crystal molecules is released without application of an electricfield whereby the dipole moment assumes either of the two states, i.e.,Pa in an upper direction 94a or Pb in a lower direction 94b thusproviding a bistability condition, as shown in FIG. 9. When an electricfield Ea or Eb higher than a certain threshold level and different fromeach other in polarity as shown in FIG. 9 is applied to a cell havingthe above-mentioned characteristics, the dipole moment is directedeither in the upper direction 94a or in the lower direction 94bdepending on the vector of the electric field Ea or Eb. Incorrespondence with this, the liquid crystal molecules are oriented toeither a first orientation state 93a or a second orientation state 93b.

When the above-mentioned ferroelectric liquid crystal is used as anoptical modulation element, it is possible to obtain two advantages.First is that the response speed is quite fast. Second is that theorientation of the liquid crystal shows bistability. The secondadvantage will be further explained, e.g., with reference to FIG. 9.When the electric field Ea is applied to the liquid crystal molecules,they are oriented in the first stable state 93a. This state is stablyretained even if the electric field is removed. On the other hand, whenthe electric field Eb of which direction is opposite to that of theelectric field Ea is applied thereto, the liquid crystal molecules areoriented to the second orientation state 93b whereby the directions ofmolecules are changed. Likewise, the latter state is stably retainedeven if the electric field is removed. Further, as long as the magnitudeof the electric field Ea or Eb being applied is not above a certainthreshold value, the liquid crystal molecules are placed in therespective orientation states. In order to effectively realize highresponse speed and bistability, it is preferable that the thickness ofthe cell is as thin as possible and generally 0.5 to 20 microns, furtherpreferably 1 to 5 microns.

As the bistable liquid crystal used in the liquid crystal apparatus ofthe present invention, ferroelectric chiral smectic liquid crystals maybe most suitably used, of which liquid crystals in chiral smectic Cphase (SmC*) or H phase (SmH*) are particularly suited. Theseferroelectric liquid crystals may be those described in, e.g., U.S. Pat.Nos. 4,613,209, 4,614,609, 4,622,165, etc.

Further, in the present invention, driving methods as disclosed in,e.g., U.S. Pat. Nos. 4,705,345, 4,707,078, etc. may be used in additionto those described above.

As described hereinabove, according to the present invention, it ispossible to effectively prevent the occurrence of flickering which hasbeen encountered in a drive at a low temperature when the drive systemis subjected to temperature compensation, i.e., lower frequency drivepulses are used at a lower temperature in order to compensate for atemperature dependence of a liquid crystal, whereby an improvement indisplay quality can be realized.

What is claimed is:
 1. A liquid crystal apparatus, comprising:a liquidcrystal device comprising a group of first electrodes, a group ofelectrodes intersecting the first electrodes, and a ferroelectric liquidcrystal disposed between the groups of first and second electrodesforming a picture area comprising a pixel at each intersection of thefirst and second electrodes; and drive means for sequentially applying ascanning selection signal to electrodes in said first group ofelectrodes, wherein said scanning selection signal is applied toelectrodes which are N electrodes apart (wherein N is a positiveinteger), and for applying data signals through the second electrodes toat least some of the pixels on a particular first electrode while thescanning selection signal is applied so as to first form a dark state atleast some of the pixels on the particular first electrode and then forma bright state at at least one selected pixel among the pixels on theparticular first electrode.
 2. An apparatus according to claim 1,wherein said drive means includes means for applying the scanningselection signal to the first electrodes in one scanning series so as toform one picture in N+1 scanning series.
 3. An apparatus according toclaim 2, wherein the application of the scanning selection signal Nelectrodes apart is performed at a rate of 20 or more scanning seriesper second.
 4. An apparatus according to claim 1, wherein saidferroelectric liquid crystal is a chiral smectic liquid crystal.
 5. Anapparatus according to claim 4, wherein said chiral smectic liquidcrystal assumes a non-helical molecular alignment structure.
 6. A liquidcrystal apparatus, comprising:a) a liquid crystal device comprising agroup of first electrodes, a group of second electrodes intersecting thefirst electrodes, and a ferroelectric liquid crystal disposed betweenthe groups of first and second electrodes forming a picture areacomprising a pixel at each intersection of the first and secondelectrodes; b) first means for sequentially applying a scanningselection signal to electrodes in said first group of electrodes,wherein said scanning selection signal is applied to electrodes whichare N electrodes apart (wherein N is a positive integer) in one scanningseries so as to form one picture in N+1 scanning series; c) second meansfor simultaneously applying data signals to the second electrodes insynchronism with the scanning selection signal; and d) third means forcontrolling the data signals so that a prescribed number of rightmost orleftmost electrodes among said second electrodes is supplied with datasignals so as to first form a dark state and then form a bright state atthe pixels on a particular first electrode under application of thescanning selection signal thereby forming a bright state at all thepixels formed at the intersections of the first electrodes and theprescribed number of second electrodes after the completion of one cycleof scanning of the first electrodes.
 7. An apparatus according to claim6, wherein said third means includes means for designating saidprescribed number of rightmost or leftmost second electrodes.
 8. Anapparatus according to claim 6, wherein N is an integer of 1-7.
 9. Aliquid crystal apparatus, comprising:a) a liquid crystal devicecomprising a group of first electrodes, a group of second electrodesintersecting the first electrodes and including a rightmost or leftmostsecond electrode which is wider than the other second electrodes, and aferroelectric liquid crystal disposed between the groups of first andsecond electrodes so as to form a picture area comprising a pixel ateach intersection of the first and second electrodes; b) first means forsequentially applying a scanning selection signal to electrodes in saidfirst group of electrodes, wherein said scanning selection signal isapplied to electrodes which are N electrodes apart (wherein N is apositive integer) in one scanning series so as to form one picture inN+1 scanning series; c) second means for simultaneously applying datasignals to the second electrodes in synchronism with the scanningselection signal; and d) third means for controlling the data signals sothat a prescribed number of rightmost or leftmost wider electrodes amongthe group of second electrodes is supplied with data signals so as tofirst form a dark state and then form a bright state at the pixels on aparticular first electrode under application of the scanning selectionsignal thereby forming a bright state at all the pixels formed at theintersections of the first electrodes and the rightmost or leftmostwider second electrodes after the completion of one cycle of scanning ofthe first electrodes.
 10. An apparatus according to claim 9, whereinsaid group of second electrodes includes both a rightmost and a leftmostwider second electrode.
 11. An apparatus according to claim 9, wherein Nis an integer of 1-7.